Cruz is now challenging Donald Trump for the lead in the kickoff Iowa caucus and casting himself as a more electable alternative who still shares the bombastic billionaire's anti-Washington appeal.

Cruz recently appeared to question Trump's judgment at a private fundraiser, according to audio obtained by The New York Times, while Trump called Cruz "a little bit of a maniac."

Another intriguing dynamic in Tuesday's prime-time debate in Las Vegas involves Cruz and Florida Sen. Marco Rubio. Rubio's campaign is eager to cast Cruz, who prides himself on being a conservative "truth-teller," as a politically expedient flip-flopper.